THE GEOMICROBIOLOGY BEHIND ARSENIC CYCLING WITHIN RIVERBANK AQUIFERS IN BANGLADESH

Groundwater arsenic (As) contamination is an ongoing problem in the Ganges-Brahmaputra-Meghna Delta, Bangladesh. The location and extent of As contamination is not fully explained by existing hydrological and geochemical models. Geochemical changes, such as oxidative precipitation of iron oxide minerals in aquifer sediments are caused by river water infiltrating shallow aquifers under the influence of tidal fluctuations. This process may play a large role in biogeochemical As cycling in Bangladesh by trapping and mobilizing massive amounts of As when river levels fall and rise respectively. Specifically, we predict that frequent flow reversal creates a natural reactive barrier (NRB) of iron (Fe(III)) oxides in the shallow Meghna riverbank aquifers that adsorb As during oxidizing conditions and release Fe(II) and As(III) during reducing conditions. An E-W monitoring well transect perpendicular to the river was sampled in January 2016 for aqueous geochemical constituents. At this time of year the groundwater at this site generally flows towards the river producing a gaining condition. Specific conductance, ORP, and rapidly changing concentrations of redox sensitive elements such as Fe, As, and Mn show that surface water is entering the shallow aquifer adjacent to the river banks. Aqueous phase concentration gradients in shallow sediments (1-3 m) along the riverbank suggest that Fe and Mn oxides are precipitating in this zone during the dry season and decreasing aqueous As concentrations show that they are likely adsorbing As. We predict that passive mineralization and biomineralization facilitated by microorganisms contributes to the mineralization of Fe and Mn. To test this, a vertical sediment core of solid phase Fe and As were measured with a handheld XRF within the proposed NRB. XRF data show high solid-phase concentrations of Fe, As, and Mn occur both in clay and course sand layers. We predict that there is another NRB in the coarse sand layer. This will be ascertained via mineralogical studies with X-Ray Diffraction. Sequencing of the 16S rRNA gene followed by metagenome analysis of selected microbial communities will be used to identify which organisms may be involved in forming the NRB and in As cycling within the NRB. Our results and techniques may be applicable to other areas of As contamination in S-E Asia.